The present invention relates to semiconductor processing and, more specifically, to a method of forming semiconductor structures including heterojunctions. A major objective of the present invention is to provide for the fabrication of Si.sub.1-x Ge.sub.x on Si heterojunctions with minimal misfit dislocations despite exposure of the already annealed Si.sub.1-x Ge.sub.x to high processing temperatures.
The recent fabrication of near-ideal heterojunction bipolar transistors has increased the need to understand the structural stability of Si.sub.1-x Ge.sub.x alloy thin films grown on silicon. (See 1. C. A. King, J. L. Hoyt, C. M. Gronet, J. F. Gibbons, M. P. Scott and J. Turner, accepted for publication in IEEE Electron Device Letters.) It is important to prevent strain relaxation in these pseudomorphic device structures to preserve the desired band gap and avoid recombination currents associated with closely spaced misfit dislocations. (See C. A. King, J. L. Hoyt, D. B. Noble, C. M. Gronet, J. F. Gibbons, M. P. Scott, T. I. Kamins, and S. S. Laderman, submitted to IEEE Electron Device Letters.)
Measurements to date of the generation of misfit dislocations during growth of Si.sub.1-x Ge.sub.x have focused on epitaxial layers grown by Molecular Beam Epitaxy (MBE). (See J. C. Bean, L. C. Feldman, A. T. Fiory, S. Nakahara, and I. K. Robinson, J. Vac. Sci. Technol.A, 2, 436 (1984); see also E. Kasper and H. J. Herzog, Thin Solid Films 44, 357 (1977)).
Some of these studies have used Rutherford backscattering (RBS) and x-ray diffraction to characterize the onset of strain relaxation. The sensitivity of these techniques is not sufficient to study the early stages of misfit dislocation generation. A combination of techniques has been used to characterize individual dislocations in as-grown MBE samples (See E. P. Kvam, D. J. Eaglesham, D. M. Maher, C. J. Humphreys, J. C. Bean, G. S. Green, and B. K. Tanner, Materials Research Society Symposium Proceedings Volume 104, 623 (1988); see also Y. Kohama, Y. Fukuda, and M. Seki, Appl. Phys. Lett. 52, 380 (1988)). However, little experimental data has been presented concerning the onset of dislocation generation induced by post-growth annealing and no comparison has been made between Si.sub.1-x Ge.sub.x formed by MBE and that formed by chemical vapor deposition.
Results are presented below on material grown by limited reaction processing (LRP), a CVD technique that relies on the rapid heating and cooling of the substrate to initiate and terminate growth. (See J. F. Gibbons, C. M. Gronet, and K. E. Williams, Appl. Phys. Lett. 47, 721 (1985)). These Si.sub.1-x Ge.sub.x films have been found to contain approximately 10.sup.20 oxygen atoms/cm.sup.3. The reports by Kasper and by Kvam, cited above, have shown that as-grown, MBE Si.sub.1-x Ge.sub.x films are metastable with dislocation spacings larger than predicted by equilibrium theory.